1. Field of the Invention
The present invention relates to charging lead accumulators and relates more particularly to charging such accumulators with a free or immobilized electrolyte.
2. Description of the Related Art
It is desirable to determine the state of charge of a battery before even starting to charge it, in order, on the one hand, to estimate its residual capacity, which provides information on the percentage of charge available and, on the other hand, to prevent charging a defective battery which could lead to irreversibly destroying it.
Thus, determining the state of the battery is considered essential for it to be recharged optimally under the best conditions without impairing its life or its nominal capacity according to the number of cycles. However, none of the chargers available on the market provide enough information allowing the state of the battery to be charged to be determined.
Several recharging profiles exist, but the latter do not really take account of the state of charge, they are mainly based on the reaction of the magnetic elements in relation to the voltage of the battery in question since they use either leakage transformers (single or double slope) or ferroresonant chargers whose output voltage is connected to the change in voltage of the battery.
All conventional chargers (chargers operating at the 50 Hz electrical engineering frequencies) generally have a preset timer triggered either at the first step or at the second step called Vgas. However, if a battery is slightly discharged, charging takes place during the preset time interval which can then heat the battery needlessly. It is also a fact that maintaining the overcharge factor according to the percentage of discharge is not easy.
Several presently known techniques, due to the advances made in electric vehicles and in the understanding of batteries, require the charging voltage of each element or of each monobloc and its temperature to be determined, this being so in order to determine possible discrepancies but also to offset the voltage according to temperature.
For the state of charge to be determined, it is necessary to have available sophisticated electronics measuring all the most relevant parameters (voltages, current, temperature) in order to activate the computing algorithms so as to determine the available energy as described in document FR-2 702 884 A1.
This approach, although valid within the context of an electric vehicle, is not justified in handling equipment when considering the cost of the necessary plant and equipment.
The methods of charging free electrolyte batteries are difficult to transfer to batteries with immobilized xe2x80x9cgelledxe2x80x9d electrolytes, since the transfer of energy, although identical at the start of charging for a discharged battery, becomes significantly different as soon as the battery voltage reaches its degassing voltage xe2x80x9cVgasxe2x80x9d which is an average 2.37 V/element at 30xc2x0 C.
This is because, although the increase in voltage above Vgas can be tolerated for open lead batteries, it is highly inadvisable for sealed batteries (gelled or absorbed electrolyte battery) since an increase in the potential means an evolution of gas which could lead to a deterioration in the battery or a loss of capacity.
However, as soon as an end-of-charge current of low value (variable between C5/100 and C5/200, instead of C5/20 and C/30 in the case of free electrolyte batteries) is reached, an increase in voltage is then allowed without risk of damage, but with the voltage being limited to a value Vmax (variable between 2.6 and 2.75 V/element).
Several charging profiles exist, those most commonly used being WA, WOWa, WU, WUIa, IU, IUIa, etc. However, it is advisable to have safety devices available such as charging time limiters depending on the various steps, voltage limiters, and capacity or even temperature limiters for conventional chargers not having a system available to calculate the actual capacity of the battery being charged.
The most suitable profiles for recharging gel or free electrolyte batteries seem to be the WUIa or IUIa profile with, as a variant, control (amplitude, duration) of the end-of-charge current depending on the technology employed, the desired charging time and the necessary charging factor. The role of the end-of-charge current Ia is also to homogenize the electrolyte along the plate in order to allow the end-of-charge voltages of the elements which have not been able to recover their final voltages to reach equilibrium.
Example of the IUIa profile: the latter consists of three different steps:
The first step of current I depends on the capacity of the battery.
This current must be neither too big so as not to damage the elements, nor too small so as not to penalize the charging time of the first step and consequently the total charging duration:
the first possibility is to control the charger according to the battery which is connected to it. However, batteries of different capacities cannot be connected to the same charger; consequently, the charger becomes single capacity;
a second possibility consists in determining the capacity of the battery and in matching the charging current to the capacity and to the state of charge of the battery.
The second step of voltage U depends on the type of battery in question. Also, from 2.37 V/element at 30xc2x0 C., the internal chemical reaction is enhanced by electrolysis of water, thus creating an evolution of gas. This voltage is called the degassing voltage xe2x80x9cVGASxe2x80x9d. Thus, it is inadvisable to exceed this voltage for sealed batteries otherwise the battery will be destroyed by evolution of gas outside the elements.
The third step of current Ia depends on the technology of the battery used; its amplitude and its duration depend on the capacity C5, on the discharge depth and on the desired charge factor.
1) for a gel battery, the current Ia will have an amplitude of between C5/100 and C5/200, and a duration of between 1 and 4 hours depending on the discharge depth and the required charge factor (typically the charge factor is about 1.05 to 1.07);
2) for a free electrolyte battery, the current Ia will have an amplitude of C5/20 and C/30 and a duration of between 1 hour and 3 hours depending on the discharge depth and the required charge factor (typically about 1.15).
The object of the invention is to optimize the recharging of batteries of the two technology types (sealed and PbO) with the same principles of calculating the capacity so as, on the one hand, to determine the charging current as a function of the battery capacity, but also to optimize the recharging factor in order to prevent any risk of needless heating, and, on the other hand, to provide proper recharging whatever the initial conditions, with a minimum of uncertainty by taking account of faults associated with the battery (deep discharge, unsuitable voltage, defective element, etc).
It also aims to obtain the best compromise for the overcharge factor/discharge factor, the temperature and the age of the battery so as to maintain the capacity of the battery during its life.
The object of the charging method is to improve the existing methods which do not take into account the set of parameters making it possible to optimize recharging. This guarantees full charging of the battery without impairing its life. The method used for recharging lead batteries with a liquefied electrode in addition comprises the profile for a gel battery, current pulses which make it possible to obtain a beneficial effect on the life of the elements, but it also contributes to preventing premature losses of capacity as described in the article xe2x80x9cT-AM et al, page 215, Journal of Power Source 53, 1995xe2x80x9d.
The subject of the invention is therefore a method of testing a lead battery for the purpose of charging it under optimal conditions, characterized in that it consists in testing the lead battery for the purpose of obtaining information relating to its condition by applying a test current and/or pulse thereto and by increasing the voltage at the battery terminals.
According to other characteristics of the invention:
it comprises a step of controlling the current by generating a variable-frequency function intended to produce a test cycle comprising the production of a current which increases up to a reference value chosen according to the capacity of the battery, which is held for a specific time defined by the capacity of the battery and which decreases such that there is significant excitation of the battery until a voltage close to its degassing voltage or greater than this voltage is obtained.
The subject of the invention is also a device for testing a lead battery, characterized in that it comprises a device for controlling the current comprising a variable-frequency function generator intended to produce a test cycle comprising the production of a current which increases up to a reference value chosen according to the capacity of the battery, then is held for a specific time defined by the capacity of the battery and decreases in such a way that there is significant excitation of the battery until a voltage close to its degassing voltage or greater than this voltage is obtained.
According to other characteristics:
the device is a generator of current then of voltage with a large dynamic output range;
the current generator delivers a current of variable slope according to the capacity of the battery.
The method and the associated device may be used in order to obtain information relating to the battery, for example the number of elements, the remaining capacity of the battery and the discharge level. The device and the method may also detect defects before charging is started (battery not voltage-matched, damaged element). Thus, using this diagnosis, the battery will be charged better according to its state while storing deficiences during the charging cycle.
According to one embodiment of the calculation process using current pulses, of variable amplitude but of fixed duration, this variable amplitude depends on the reaction of the battery voltage. The calculation determines the current at the start of charging after diagnosis, before charging, is started, in order to prevent recharging an unsuitable battery.
During this phase, which is common to both types of technology, the battery state of charge is determined after four to five series of current pulses. This number may vary depending on the battery type: for example, for a gelled electrolyte battery, this number is for example 4, but for free electrolyte batteries, this number may for example exceed 5 for high capacities.
The effects of the first group of pulses is to stimulate the battery, and to prevent certain random phenomena associated with the electrochemical process during reactions. The measurements for estimating the capacity will be made only after a certain time (the time needed to avoid the time constants of the chemical reactions of the positive electrode, and of the negative electrode with the electrolyte). This delay in measurement for a time Tr also makes it possible to avoid transient phenomena which could occur at the start of charging and cause errors in measurement.